1. Field of the Invention
The invention relates generally to digital signal transmissions and particularly to frequency-modulated radio frequency transmissions of digital signals.
2. Discussion of the Prior Art
Frequency modulated ("FM") radio technology for voice communications is well established. Carrier frequencies are typically generated by crystal oscillator circuits. Such basic crystal oscillator circuits are coupled to or integrated with well known and commercially available modulation circuits. Crystals are known elements for sustaining oscillations at characteristic frequencies in such well known circuits. Even though, present communications standards are of such stringency that temperature compensation networks become necessary to enable such circuits to maintain an established center frequency over a specified temperature range. Thermistor-based compensation networks are typically used to stabilize carrier frequencies of crystal oscillator circuits over typically specified temperature ranges, such as from negative thirty degrees centigrade to positive sixty degrees centigrade.
A typical FM modulating circuit includes a series coupled combination of a capacitor and a varactor diode. The varactor diode of such circuit is reverse-biased to ground and its cathode is coupled to one terminal of the capacitor. The second terminal of the capacitor is coupled to the crystal oscillator circuit such that the equivalent capacitance of the series-coupled varactor diode and capacitor combination become a frequency determining capacitance element in the crystal oscillator circuit. The node between the varactor diode and the capacitor is adapted to receive a voltage type signal input for modulating the center frequency output of the crystal oscillator circuit. Variations in voltages of input signals to the node results in corresponding shifts in the reactance of the varactor diode and, hence, the frequency of oscillation of the oscillator circuit. In the absence of a modulating voltage shift at the node, a steady state voltage at the cathode of the reverse-biased varactor diode constitutes a signal voltage reference at which the crystal oscillator circuit oscillates at its unmodulated center frequency. In that such voltage at the signal input node is critical to the stability of the center frequency of the crystal oscillator, a circuit for stabilizing typical frequency drifts over an operational temperature range is directly coupled to the node between the varactor diode and the capacitor. Such a temperature compensation circuit changes the voltage at the cathode of the varactor diode over the effective temperature range in such a manner that the center frequency output of the oscillator circuit remains stable over such range. In operation of the described circuit, a voltage modulation input to the node, such as the electrical output from a microphone in response to its reception of sound or speech, results in a correspondingly modulated frequency output of the crystal oscillator circuit.
Voice signals typically have no DC component. Such signals can consequently be superimposed through a series-coupled input capacitor on the established, temperature compensated voltage at the node. Without a DC current input to the node the reference voltage at the node and, hence, the center frequency of the circuit remain stable. If capacitive coupling is not used, and the signal impressed on the input node includes a DC component which differs from the temperature-compensated steady state reference voltage at the node, the reference voltage at the node would be shifted. Such shift, in turn, would shift the center frequency of the transmission signal with respect to which the modulated signal is centered. Depending on the magnitude of the voltage shift, the modulated frequencies may become shifted beyond allowable limits, such as, for example, those established by the Federal Communication Commission.
Digital data signals typically include a DC component which causes such signals applied through the referred-to capacitive coupling to become distorted with what is referred to as "jitter". In fact, data signals will vary in their DC component in an unpredictable manner, depending on the sequence of data presented for transmission. To control distortion of digital data signals which are applied to the modulation circuit through a capacitively coupled terminal, electronic transformation or encoding of data pulses is used. According to one particular technique referred to as biphase coding, each signal pulse is split into two components. A positive pulse or "one"-pulse may be split into a first, positive signal component and a second, negative signal component. Conversely, a "zero"-pulse, also referred to as ground or negative pulse, may be represented by a first, negative component, followed by a second, positive component. In such an encoding scheme each pulse, whether positive or ground, and consequently any combination or string of data pulses, would be balanced about a ground datum and may then be applied through a capacitor to the referred-to modulation circuit. A problem is likely to be encountered, however, in decoding the encoded signals, particularly in synchronizing the decoding circuits to allow them to decipher the received transmissions. A loss or addition of a single bit would be likely to cause an entire data transmission to become scrambled.
A further disadvantage of the previously described coding or modulation method is that two modulation transitions are required to transmit each binary bit of information. Since data signalling speed is typically limited, such as by the channel bandwidth, the described coding or modulation method reduces the maximum available data transmission rate to one half of the otherwise available rate.
Consequently, it would be desirable to be able to transmit digital signals over traditional voice frequency transceiver units without a need for encoding and subsequently decoding the data signals and not to be concerned about eliminating DC components from a digital data string.